# Mirror Effect of Parvalbumin and Connexin 43 Expression in the Acute and Subacute Phases After Penetrating Traumatic Brain Injury Reveals a Non-Canonical Interaction

## TL;DR

This study shows how the proteins Parvalbumin and Connexin 43 change in opposite ways after brain injury, highlighting their role in calcium and pH-dependent interactions.

## Contribution

The study reveals a novel phase-dependent mirror effect of Parvalbumin and Connexin 43 expression and their Ca2+- and pH-dependent interaction after TBI.

## Key findings

- 24 hours after TBI, Parvalbumin increases with nuclear translocation while Connexin 43 decreases significantly.
- Seven days post-injury, Parvalbumin levels drop while Connexin 43 increases and forms cluster-like structures.
- Molecular simulations show that Parvalbumin–Connexin 43 stability depends on Ca2+ and pH, with acidosis and overload causing destabilization.

## Abstract

Traumatic brain injury (TBI) initiates a cascade of molecular and cellular reactions leading to long-term disturbances of neuronal and glial homeostasis. One of the key mechanisms of secondary injury is a pathological increase in intracellular Ca2+ concentration. Parvalbumin (PV) plays an important role in the regulation of Ca2+ homeostasis in neurons. In turn, connexin 43 (Cx43) is the principal protein of astrocytic gap junctions (GJs), which ensure neuroglial communication. The spatiotemporal changes in these proteins and the mechanisms of their interaction after TBI remain insufficiently studied. In the present study, a comprehensive analysis of the expression, localization, and spatial organization of PV and Cx43 in the cerebral cortex following TBI was performed. In intact tissue, PV was localized predominantly in neurons, whereas Cx43 formed typical punctate structures of astrocytic GJs. Twenty-four hours after TBI, a sharp activation of PV with pronounced nuclear translocation was observed against the background of a catastrophic decrease in Cx43 expression, accompanied by a reduction in the number of NeuN+ neurons and signs of apoptosis. However, after 7 days, a mirror-opposite effect was detected, characterized by decreased PV expression and increased Cx43 levels with its aggregation into cluster-like structures, as well as partial restoration of NeuN immunoreactivity. In addition, molecular dynamics simulations demonstrated that the stability of the PV–Cx43 complex is determined by the presence of Ca2+ and physiological pH, whereas acidosis and Ca2+ overload destabilize their interaction. Taken together, these results reveal a phase-dependent mirror-opposite pattern of PV and Cx43 expression and localization and emphasize the key role of Ca2+- and pH-dependent neuroglial interactions in TBI.